In a conventional electronic sphygmomanometer, a blood pressure has been measured by fastening a cuff to an upper arm, but such measurement has been troublesome because it is required to roll up a sleeve for cuff fastening. To avoid this trouble, there has been recently proposed an electronic sphygmomanometer of a type where a cuff is fastened to a wrist. However, unlike an upper arm, a wrist has a plurality of main arteries, various pulse waves are detected depending on a cuff fastening state, resulting in poor reproducibility. Therefore, a blood pressure cannot sometimes be measured precisely depending on how to mount the cuff.
That is, like a sphygmomanometer of an arm type where the cuff is fastened to the upper arm, such sphygmomanometer of arm type where the cuff is fastened to the wrist inhibits blood stream and measures a blood pressure by compressing arteries by the cuff, and a wrist has a radial artery and an ulnar artery, and has radius and ulna bones or the like around these arteries. Thus, when a wrist is compressed by the cuff, the compressive pressure of the cuff compresses these two arteries via wrist tissues including these bones.
The pressure transmission efficiency for the cuff to compress arteries is determined depending on an artery position (a depth from a wrist surface) the artery or a positional relationship between and wrist tissues (in particular, bones or cords), and therefore, the substantial compressive pressures applied to both arteries are not always the same. In the case where the compressive pressures applied to the arteries are different from each other, a pulse wave amplitude of an artery with its poor transmission efficiency is generated as a first peak on a high-pressure side, and the pulse wave amplitude of an artery with its good transmission efficiency is generated as a second peak on a low-pressure side. Therefore, the pulse waves detected by pulse wave detecting means detects composite waves of the pulse wave amplitude of both arteries, resulting in detecting two peaks. In addition, the transmission efficiency of the compressive pressure of the cuff varies depending on a method for fastening the cuff and subjects, and thus, the obtained pulse wave amplitude is disperse and diversified, resulting in poor reproducibility.
Thus, in the case where a first-peak pulse wave is included in the pulse wave detected by the pulse wave detecting means, when the obtained pulse wave amplitude is used as it is, to determine a blood pressure value, the blood pressure value becomes high, which causes dispersion of the blood pressure values.
In addition, according to the results of investigation of 10 subjects which the inventors made, the subjects who tend to have two peaks of pulse waves, which are prone to be generated during blood pressure measurement, irrespective of the cuff fastening position, the pulse wave of the first peak was observed at a high frequency in the cuff pressure indicating the amplitude value of 50 to 80% of the maximum amplitude pulse wave on the high pressure side rather than the maximum amplitude pulse wave.
Further, the pulse wave of the first peak is detected in the case where the pressures applied to radial and ulnar arteries are substantially different from each other, but is not detected in the case where the pressures applied to both of the arteries is identical to each other. In addition, in the case where the first and the second peaks are detected immediately after only the second peak has been detected, the reproducibility of the pulse wave of the first peak is impaired.
On the other hand, the amplitude of the pulse wave of the second peak is often the maximum amplitude in general, and the pulse wave amplitude has reproducibility.
Comprehensively considering these findings, in the electronic sphygmomanometer fastening the cuff to a wrist, it is necessary to cancel the pulse wave amplitude of the first peak with poor reproducibility and calculate a blood pressure value in order to measure a precise blood pressure value with a good reproducibility.
It is one object of the present invention to provide an electronic sphygmomanometer capable of performing precise and reproducible blood pressure measurement even in the case where distortion occurs with detected pulse waves depending on how to mount the cuff as in an electronic sphygmomanometer, and in particular, a sphygmomanometer of cuff type where the cuff is fastened to the wrist or in the case where there is certain distortion with the detected pulse waves depending on the physical conditions of a subject; and a method for measuring a blood pressure using this sphygmomanometer.